Handbook for Methane Control in Mining - AMMSA

142Figure 11–1.—Desorption test apparatus.LaScola et al. [1981] alsocollected conveyor beltgrab samples. Since thecoal in the silos they investigatedwas usually storedin the silo for about 24 hr,they used the 24-hr cumulativeemission value as acomparative index. These24-hr emission rates rangedfrom 3.3 to 86 ft 3 /ton.LaScola et al. noted that allof the mines with 24-hremission values exceeding14 ft 3 /ton had open-topsilos to provide better ventilationat the top. Also,most had forced ventilationof the reclaiming areas,at ventilation rates rangingfrom 5,600 to 20,000 cfm.Kolada [1985] reportedsimilar gas amounts fromsilo conveyor belt samplesat Canadian mines. 7Interestingly, grab samplesfrom coal entering cleancoal silos sometimes gaveemission rates five timeshigher than coal enteringraw coal silos. This is notwhat one would expectsince clean coal has beenFigure 11–2.—Gas emitted from conveyor belt grab samples.out of the mine longer.However, in theseinstances, the clean coal had been passed through a fluidized bed dryer to remove moisture andits temperature was 40 °C as it was being loaded into the silo. The higher temperature greatlyincreased the methane emission rate. 87 More information on this study is available from AMCL [1985].8 The breakage of coal during the cleaning process could also have been a contributing factor to the elevated methaneemission. Friable coals will fracture into smaller size particles during coal cleaning, and smaller size particles willgive off methane more quickly [Mikhail and Patching 1980].